1. Introduction
Mitochondria contain their own genome, a circular double-strand DNA
molecule (mtDNA) of 16,569-bp that encodes for proteins of the
mitochondrial respiratory chain and mitochondrial tRNAs and rRNAs.
Because mtDNA is inherited from the mother, germ-line mutations in the
mtDNA are associated with rare maternal inherited diseases. The mtDNA is
prone to acquire nucleotide changes that accumulate with age or under
exposure to environmental toxics [Lee et al., 1998;Wallace 2010; Bratic and Larsson , 2013; Ziadaet al., 2019].
In addition to rare pathogenic variants the mtDNA contains many common
variants that originated in individuals from particular populations and
spread with worldwide migrations. Specific combinations of these
variants classify the mitochondrial haplogroups, with frequencies that
are characteristic of each human population [Wilson and
Allard, 2004; Torroni et al., 2006; Brotherton et al., 2013] .
For instance, haplogroup H is defined by 7028C (among other nucleotide
changes) and is the most common among Europeans while is absent among
individuals of African or East Asian ascent. These mtDNA haplogroups are
transmitted from mother to offspring and their worlwide distribution
permitted to trace the migration of humans outside Africa, raising the
concept of a mitochondrial Eve [Pakendorf andStoneking , 2005].
Mitochondrial DNA variants/haplogroups have been associated with
differences in physiological processes such as energy production,
ageing, regulation of apoptosis or pathogen immune-mediated responses
[Gómez-Durán et al., 2010; Chen et al., 2012;Kenney et al., 2014; Krzywanski et al., 2016; Friedrich et al.,
2022]. As a consequence, these haplogroups have been associated with
adaptation to exercise or susceptibility to develop several traits such
as diabetes, cardiovascular disease, or infectious diseases [Castro et
al., 2007; Yonova-Doing et al., 2022]. In reference to
infections these variants might be associated with the risk of sepsis or
the severity of HIV and herpex disease, among others
[Hendrickson et al., 2008; Yang et al., 2008; Hart et al.,
2013; Levinson et al, 2016 ]. Haplogroups might also play a role in
the risk for severe COVID-19, the disease caused by SARS-CoV-2
[Wu et al., 2021; Dirican et al., 2022; Vázquez-Coto
et al., 2022; Kumari et al. 2023].
Each cell has a variable number of mitochondria and each mitochondria
contains several copies of the mtDNA. For a particular nucleotide
position, the mitochondria from each individual may exhibit the same
variant (homoplasmy) or different alleles (heteroplasmy)
[Santos et al., 2008; Li et al., 2010;Klütsch et al., 2011]. Heteroplasmy is commonly inherited
from the mother and for disease-related variants the degree of
heteroplasmy in the different tissues determines the extent of the
symptoms. Rare highly penetrant mutations cause monogenic disorders that
often affect the nervous system, muscles, heart, and endocrine organs,
and many healthy individuals carry low levels of heteroplasmy
(<1% of mtDNA with the mutation) either inherited from the
mother or acquired. An increased burden of heteroplasmy contributes to
increased risk for diseases such as MELAS (mitochondrial
encephalomyopathy, lactic acidosis and stroke-like episodes), diabetes
mellitus, and others [Avital et al., 2012; Folmes et
al., 2013; Chae et al., 2020].
Heteroplasmy is common in poly-cytosine tracts in the mtDNA control
region [Bendall et al., 1995; Lagerström-Fermér et
al., 2001; Shin et al., 2006; Zhao et al., 2010;Mueller et al., 2011; Shen et al., 2015]. One of
these is located between nucleotides 16184-16193, that contains the
origin for replication of the mtDNA heavy (H) chain. These poly-C tracts
are prone to length instability that would increase the risk of mtDNA
loss and impairment of mitochondrial regulated processes
[Chiaratti et al., 2022]. Among others, the T16189C mtDNA
polymorphism increases the risk for poly-C instability and has been
associated with diabetes, cancer, and coronary artery disease (CAD),
among other diseases [ Zhao et al., 2010; Mueller et
al., 2011; Shen et al., 2015]. Length heteroplasmy might be
at low level in resting while increases in cells subjected to extensive
division, such as the immune cells under chronic inflammation. Increased
heteroplasmy might thus represent a marker of a deleterious
immune-response that would increase the risk of developing severe
infectious diseases [Stefano et al., 2022; Ren et
al., 2020; Elesela et al., 2021; Li et al., 2021].
Due to the overactivation of the immune system among individuals
infected by SARS-CoV-2 we hypothesised that blood leukocytes from
patients with severe COVID-19 might exhibit a different profile of
poly-C heteroplasmy. To address this issue we characterised the mtDNA
region containing the 16184-16193 poly-C tract in patients with critical
COVID-19 and age-matched population controls.